A) Field of the Invention
This invention relates to a solid-state imaging apparatus and its driving method, and more in detail, relates to a solid-state imaging apparatus and a driving method of the solid-state imaging apparatus for preventing damage in a dynamic range of the solid-state imaging device.
B) Description of the Related Art
FIG. 6 is a schematic plan view showing a conventional solid-state imaging apparatus 51.
The solid-state imaging apparatus 51 is an inter-line type CCD (ITCCD) that is the most widely used as the conventional solid-state imaging apparatus. A large number of photoelectric conversion elements (pixels) 52 are arranged in a tetragonal matrix in a light receiving region 62. In each interval of the rows of the photoelectric conversion elements 52, a vertical signal charge transfer circuit (VCCD) 64 that read the signal charge generated by the photoelectric conversion elements 52 to transfer in vertical direction are formed including a transfer electrode and a vertical transfer channel, and the signal electric charge generated by the photoelectric conversion elements 52 is transferred in vertical direction.
In the drawing, a horizontal electric charge transfer circuit (HCCD) 73 that transfers the electric charge transferred by the VCCD 64 to a peripheral circuit 75 by one line is formed.
FIG. 7A is an enlarged plan view showing a part of the light receiving region 52 of the conventional solid-state imaging apparatus 51. FIG. 7B is an enlarged cross sectional view of a broken line A to B in FIG. 7A. FIG. 7C is an enlarged cross sectional view of a broken line C to D.
Each of the vertical transfer channel 54 is formed corresponding to each row of the photoelectric conversion elements 52, and transfers the signal electric charges read out via a reading-out gate channel region 51c formed adjoining to each photoelectric conversion element 52 to the vertical direction. A channel stop region 53 is positioned adjoining to the vertical transfer channel 54 on the opposite side of the reading-out gate channel region 51c. Moreover, the transfer electrodes 56 (the first layer poly-silicon electrode 56a and the second layer poly-silicon 56b) are formed over the vertical transfer channel 54 via the insulating film 60a. 
During a reading-out period, the signal charges generated by the photoelectric conversion elements (pixel) 52 are read out to the vertical transfer channels 54 by imposing a high level voltage (VH) to the second layer poly-silicon electrode 56b (φV1) equipped on the reading-out gate channel region (reading-out part) 51c. 
Thereafter, during a transfer period, the signal charges are transferred to the HCCD 73 by sequentially imposing a mid-level pulse (VM) or a low-level pulse (VL) to the transfer electrodes 56a to 56d. A horizontal transfer of the electric charges by the HCCD 73 is executed by the two-phase drive with HM/HL pulses during a period between the transfer operations of the VCCD 64 in the transfer period.
FIG. 8A shows electric potentials between a broken line E-F in FIG. 7B. An overflow drain that discharges an excessive signal electric charge of the photoelectric conversion elements 52 is formed by adding an inverse bias on an n-type substrate 51a to form an appropriate electric potential barrier between the photoelectric conversion element 52 and the n-type substrate 51a. 
In the drawing, the electric potential indicated with a solid line is in a condition that the electric charges are accumulated in the photoelectric conversion element 52. Since a low voltage (VM or VL) is imposed on the electrode 56b, a reading part 51c is closed, and the accumulated signal charges are not read out to the vertical transfer channel 54.
In the drawing, the electric potential indicated with a dashed line is in a condition that a high voltage (VH) is imposed on the electrode 56b, and the electric potential barrier to the vertical transfer channel 54 from the photoelectric conversion elements 52 is eliminated by imposing a sufficient high voltage, and all the electric charges will move to the vertical transfer channel 54.
This is that the photoelectric conversion element 52 is in a fully-depleted condition. If the full-depletion is not executed at a time of reading out, for example, the electric charge left in the photoelectric conversion element 52 is added to the signal electric charge stored until the reading-out time in a case that the reading-out operation and the transfer operation are continuously repeated as movie motion, and a remarkable picture degradation of movie picture is caused as an after image phenomenon. Therefore, it is necessary that sufficient voltage is imposed for the full-depletion of the photoelectric conversion element 52 at a time of reading-out. Hereinafter, the minimum necessary voltage for the full-depletion is called the minimum full-depletion voltage.
FIG. 8 shows the electric potential between a broken line G-H in FIG. 7C. In the drawing, a solid line shows a state that a low voltage (VM or VL) is imposed on the electrode 56b, and a broken line shows a state that the high voltage (VH) is imposed on the electrode 56b. 
Since the electric potential of the reading-out part 51c is low in the adjusting electrode 56a and the channel stop region 53, a potential slope is generated by a fringe electric field. For example, as shown in the drawing, the electric potential in a lower part of the electrode 56b will be an electric potential distribution with an acute angle, and it will be a low electric potential as compared to the case that a width of the electrode is sufficiently wide. This is so-called a narrow channel, and it is necessary to impose higher voltage for obtaining desired electric potential. That is, it causes the minimum full-depletion voltage.
Moreover, since it is desired that a film thickness of an insulating film 60a is made thin for increasing transfer capacity per unit area of the VCCD 64 in miniaturization of the element, it is necessary that the minimum full-depletion is controlled to be low. Moreover, when the high voltage is necessary, increase in the consumption of electricity will be caused.